Recording - March 2014 - 52

By Bruce Black
Over the time that I've been doing acoustical
work, I've found that there are a number of misconceptions surrounding just what speaker isolators are, what they're supposed to give you,
how they work, and why one kind works better
than another. Common wisdom says you just
throw a slab of rubber or foam or something
under your speaker, and like Merlin waving his
wand, it "makes your speakers sound better."
Wrong-there's a lot more to it than that. By
understanding what is going on, you can install
speaker isolation that is truly effective, and costs
the least. So let's deconstruct this.
The problem
Low frequencies require lots of Watts to move
the cone that moves the air that makes the SPL
that Jack wants. No speaker system is 100%
efficient in transferring electrical power to
acoustical power, so some of the juice from the
power amp ends up transferring to the speaker
cabinet as kinetic energy (i.e., vibration).
This kinetic energy migrates from the cabinet
to whatever it is sitting on, and through that into
the entire room structure. Energy then transfers
along all the points of solid contact ("interfaces") in the room's structure, by virtue of one
object being in tight contact with another. This
transfer is very efficient because the weight and
contact surface area of the speaker keeps it
tight to the surface it sits on, and nails and
screws tightly clamp all the pieces of the building structure together. These interfaces have
very little attenuation, providing a path for the
kinetic energy to infiltrate the entire building.
A very simple device demonstrates this:
Remember an executive toy called Balance
Balls (Figure 1)? It's called Newton's Cradle
in scientific circles, and demonstrates the law
of conservation of energy and momentum
(en.wikipedia.org/wiki/Newton's_cradle).
Five balls are suspended in a cradle; if you
lift and release the ball at one end, the ones
in the middle remain still, while the one at the
opposite end flies out. Like magic! This is how
sound energy can permeate through a building structure with almost no attenuation.
This energy finds release in room surfaces.
No drywall or paneling surface is completely rigid, so when this energy reaches them,
they vibrate. This, in turn, makes the air
vibrate-which, as we all know, is sound.
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RECORDING March 2014

The effect
And so the trouble begins. The first thing you find out is how well the room surfaces are attached to their framework. Any loose drywall, plywood, or similar panels will buzz, and loose lighting fixtures and decorations will rattle.
Each panel vibrates at a resonant frequency, so specific frequencies are
enhanced. This acts like a mechanical equalizer that changes the response of your
room, in different ways in different locations in the room. And don't bother trying
to fix it with EQ in your audio path-you're working on the wrong thing.

But there's more. When this sound combines with the direct sound from the speakers, you get a comb filter effect from the two waveforms combining. This, too, alters
your room's frequency response. Since each point in the room will have a different
response due to amplitude and phase/time differences between the two signals,
you can only equalize from a vague point called the "listener's position", or average multiple positions, and hope the rest of the room kind of follows along.
Unfortunately most rooms aren't this cooperative.
But wait! There's even more. From the listener's perspective, the sound from this secondary source comes from a different direction, so it can alter the "real" sounds' position in the sonic panorama, smearing its localization in a way that varies according
to where you're standing. This is can be particularly noticeable when panning a
sound or with a phantom center, as these require good localization to be effective.
All this from one vibrating panel! Now add in more vibrating surfaces, multiple speakers all blaring away at once and the ever-present room resonances. It's easy to see how
this can become a complex, messy situation. And then there's sound intrusion-what happens when this kinetic energy finds its way into the rest of your building through walls and
floors to come out in other rooms...!
Breaking the chain
As you now know, the path for kinetic energy from your speaker to your room
surfaces is a chain of multiple interfaces. If you could completely break just one of
the interfaces in this chain, the problem would be solved.
In situations like this, the simplest and most effective solution is to isolate the
source from its supporting surface. In our case, this is the speaker cabinet. And
what's nice about this is it can be done quickly and inexpensively.
It ain't heavy-it's my isolator
A number of different ways have been tried to isolate a speaker cabinet. One is to
put something really heavy between your cabinet and whatever it sits on. This is called
an inertia block. The idea behind the inertia block is that the more something weighs,
the more energy it takes to move it. This is Newton's First Law in practice-a body at
rest stays at rest until an unbalanced force acts upon it, i.e. more weight requires more
force to overcome its inertia.

Recording - March 2014

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